Starting in early May 2020, a new version of the StatsPlus Desktop app is being released to move on from APIs that Slack no longer allows.
After the commissioner or simmer uses the Desktop App to install the "StatsPlus v2.0" Slack App to your league workspace, anyone with a private score channel needs to invite the app to that channel.
The easiest method to invite the app to the channel is to go into the channel and run a Slack slash command to invite the app:
/invite @StatsPlus
After you've done this, if your channel was previously set up with an older version of the desktop app, it should be re-connected automatically. If not, you will need to let the commissioner know the channel name so it can be connected for you.
Alternate steps to invite (may not be working as of Jan 2021):
Starting in January 2021, a number of users noticed that the "Add" option was no longer available on the list of apps, but the slash command method still works.